Bucket Assignment Policy
The bucket policy uses the URI of a document to decide which forest the document should be assigned to. The URI is first "mapped" to a bucket then the bucket is "mapped" to a forest. The mapping from a bucket to a forest is kept in memory for fast access. The number of buckets is always 16K, regardless of the number of forests in the database.
Note
How document URIs are mapped to buckets and buckets are mapped to forests are non-configurable implementation details.
Though there are 16K buckets used by the bucket assignment policy, for the purposes of the example illustrated below, assume there are eight buckets that distribute the 1200 documents across three forests: ForestA, ForestB, and ForestC and that the document URIs allow for even distribution of them among the buckets. ForestD is then added to the database and the rebalancer moves 1/3 of the documents from Forests A and B to ForestD by reassigning Bucket 3 from ForestA to ForestD and Bucket 6 from ForestB to ForestD.
The bucket assignment policy is, in most situations, the most efficient document assignment policy because it is deterministic and it moves the least amount of data of the deterministic assignment policies.